Device for optical coupling of a solid-state laser with an optical wave guide and a process for their production

ABSTRACT

In a device for optical coupling of a solid-state laser to an optical fiber, an anamorphic lens system is arranged between the output face of the solid-state laser and the input face of the optical fiber, the lens system converting differing apertures in the main sections of the solid-state laser into essentially identical apertures at the input face of the optical fiber. The lens system is composed of one anamorphic lens applied on the output face and a further lens applied on the input face.

FIELD OF THE INVENTION

The invention relates to a device for the optical coupling of asolid-state laser to an optical fiber, an anamorphic lens system beingarranged between the output face of the solid-state laser and the inputface of the optical fiber and converting differing apertures in the mainsections of the solid-state laser into essentially identical aperturesat the input face of the optical fiber, and to a method for itsmanufacture.

For efficient coupling of laser light into optical fibers, particularlyinto single-mode optical fibers, it is necessary to achieve minimum-losscoupling of the optical fiber to the laser. This can be effected bylenses applied to the fiber end. In the case of solid-state lasers, itis also desirable to minimize to the greatest extent possible the lossof light caused by coupling the laser to the fiber. For this purpose, ithas become known, for instance, from R. Zengerle, H. J. Bruckner, H. W.P. Koops, H.- J. Olzhausen, G. Zesch, A. Kohl and A. Menschig in“Fabrication of Optical Beamwidth Transformers for Guided Waves on InPUsing Wedge-Shaped Tapers”, J. Vac. Sci. Technol. B9(6), (1991) 3459, touse lithography to define and integrate a made-to-order coupling taperon the laser directly in the laser material. Such tapers, however, areonly able to adapt the wave field to the phase in one section. Owing tothe epitactic growth and the plane lithography used for the structuring,no adaptation is possible in the other section lying normal to thewaveguide.

In addition, a device as set forth by the species defined in the MainClaim is known from U. Griebner, R. Grunwald, H. Schönnagel, OSAProceedings on Advanced Solid-State Lasers, 1995, Vol. 24, 253, which,however, requires considerable expenditure for adjustments. Deviceshaving in each case several lenses to be adjusted and fixed in positionare also described in U.S. Pat. No. 5,140,608, British PatentApplication No. GB 2 220 501 A, German Patent Application No. DE 39 19484 A, European Patent Application No. EP 0 484 276 A, Dutch PatentApplication No. NL 8600844 A and European Patent Application No. EP 0706 070 A.

The object of the present invention is to propose a device for couplinga solid-state laser to an optical fiber, which permits substantiallyno-loss coupling and an adaptation of the wave fields in the directionof both main sections, and which can be manufactured with the necessaryaccuracy.

This objective is achieved according to the invention, in that the lenssystem is composed of one anamorphic lens applied on the output face anda further lens applied on the input face.

The device of the present invention has the advantage that in each case,the lenses can be applied on the output face and input face,respectively, with very high accuracy. An adjustment is then onlynecessary between the laser axis and the axis of the optical fiber. Boththe anamorphic lens and the further lens can be advantageouslymanufactured using well-known processes.

It may be that an anamorphic lense on the input face is not ruled out inthe device of the present invention, however, the further lens ispreferably spherical. Moreover, this specific embodiment of theinvention permits two designs, namely that the anamorphic lens appliedon the output face of the laser is an elliptical lens, or is formed oftwo crossed, cylindrical partial lenses. The elliptical lens or at leastone of the component lenses may be designed as a Fresnel lens.

The advantageous specific embodiment of the device according to theinvention can further be designed in such a way that a spacer layer,corresponding to the magnification necessary in the direction of thelarger aperture, is provided between the anamorphic lens and the outputface.

Besides a real imaging of the output pupil of the laser, the device ofthe present invention makes it possible for the output pupils of thelaser to form a virtual circular source image. This measure makes itpossible to select a small distance between the lenses or between theoutput and input faces, accompanied by relatively great focal lengths ofthe lenses. The great focal lengths of the lenses, in turn, are morefavorable for production using microtechnique methods.

Various well-known methods are suitable in principle, such as thedefinition of the cylinder lenses on the input face of the optical fiberwith the aid of high-resolution electron-beam lithography and subsequentfabrication by reactive dry etching. Such a method is described, forinstance, in “High Resolution Electron Beam Lithography for FabricatingVisible Semiconductor Lasers with Curved Mirrors and IntegratedHolograms” by P. Unger, V. Boegli, P. Buchmann and R. Germann,Microelectronic Eng. 23, (1994) 461 and in “Fabrication of curvedmirrors for visible semiconductor lasers using electron-beam lithographyand chemical assisted ion-beam etching” by P. Unger, V. Boegli, P.Buchmann and R. Germann, J. Vac. Sci. Technol., B. 11(6) (1993)2514-2518. Machining from resist lenses applied on the input or outputface is also possible.

However, a particularly advantageous method for producing the device ofthe present invention is to define the lenses and/or spacer layersand/or antireflection layers with the aid of a dry-resist technique andto fabricate them with the aid of additive lithography, in particular,electron-beam lithography. This method represents a substantialimprovement, especially if it is supported by computer programming.

The dry-resist technique, described, for example, in the German Patent195 31 859.5 A1, makes available a method in which, using vapordeposition in high vacuum, the laser or the fiber end is covered with adefined layer thickness of a polymer which is sensitive to electrons.This polymer is cross-linked by the electron beam during the exposure toform a polymer which is rich in silicon oxide and whose refractive indexis well-matched to that of the fiber material (n=1.48), see H. W. P.Koops, S. Babin, M. Weber, G. Dahm, A. Holopkin, M. Lyakhov, “Evaluationof Dry Resist Viny-T8 and Its Application to Optical Microlenses”,Microelectronic Engineering 30 (1996), 539. A mirror composed of siliconoxide is applied by vapor deposition to the laser end, the refractiveindex of the lens material likewise being well-matched to that of themirror. Thus, the insertion loss of these lenses manufactured from dryresist can theoretically be disregarded.

Using additive lithography with electron-beam-induced deposition, thelenses are directly constructed from precursor molecules, adsorbed fromthe vapor phase, by electron-beam polymerization and cross linking,using computer control of the beam and the dose, without any need for aprevious coating or subsequent development of the structure. Such amethod is described by H. W. P. Koops, R. Weiel, D. P. Kern and T. H.Baum in “High Resolution Electron Beam Induced Deposition”, J. Vac.Sci., Technol. B 6(1), (1988), 477. However, the exposure time needed isvery much greater compared to the resist technique, but remains withinjustifiable limits per lens.

In addition, by the use of the easily controllable electron beam in thescanning electron microscope, the placement of the exposure fieldrelative to the fiber core and the effective zone of the laser ispossible with an up to 100 nm precision, using image processing andscanning microscopy, which is described, for example, by H. W. P. Koops,J. Kretz, and M. Weber in “Combined Lithographies for the Reduction ofStitching Errors in Lithography”, Proc. EIPB94, J. Vac. Sci. Technol. B12 (6) (1994) 3265-3269. Due to the macrocontrol of the adjustment andthe exposure, the exposure process can be automated, controlled by aprogram.

By the computer control of the exposure and the precalculation of thedose distribution according to measured gradation curves of the resistor deposition process, besides round, elliptical, spherical andhyperbolic lens combinations, lens combinations which are provided witha deviation prism and guide the laser beam so as to orient it can alsobe jointly realized and precisely adjusted in a refractive surface. Theadjustment and the manufacturing process are combined in one, and aresuperior to conventional methods by at least one order of magnitude. Atthe same time, the easy controllability and image rotation during theelectron-beam exposure represent a method for constructing the lenseswhich is superior, for example, to laser ablation. These procedures canbe easily automated for productive use.

Exemplary embodiments of the invention are shown schematically in theDrawing with the aid of several Figures, and are explained moreprecisely in the following description.

FIG. 1 shows an examplary embodiment in the y-z section;

FIG. 2 shows the exemplary embodiment in the x-z section in anappropriately modified scale; and

FIG. 3 shows another exemplary embodiment in the z-z section.

Located in a plane 1 is an anamorphic plano-convex lens 2 with focalpoints F_(y), and −F_(y). The light output face of a solid-state laser,which is otherwise now shown, lies in plane 3. The source image islocated at 4, a virtual magnified imagery of the source image resultingat 5.

Since in the y-z section (FIG. 1), the light emerging from output face 3is virtually unfocussed, a large aperture is necessary to collect aslarge a portion of the total luminous flux a possible which, in theexemplary embodiment, is achieved by arranging a lens which is not toolarge as close as possible to output face 3 - namely, as close aspossible for a virtual imagery.

The optical fiber, only partially indicated, is composed of an opticallyactive core 6 and a cladding 7. Applied on light input face 8 is aspherical lens 9 which creates in core 6 a real image of virtual imagery5.

For the sake of clarity, the schematic representation is not true toscale. The following dimensions are given as examples:

The focal length of the lens, and simultaneously the approximatethickness of a spacer layer between lens 2 and plane 3, is τ=1.43 μm,with a refractive index of anamorphic lens 2 of n=2.75 and a radius ofR=3.9 μm. The width of the output face is d₀=0.3 μm, while the diameterof core 6 is d₁=10 μm. The apertures are a₀=45° and a₁=10°, themagnification is V=14=B/g=20/f.

FIG. 2 shows the same exemplary embodiment in the x-z section. Mountedhere on the output face of the laser is a lens having a long focallength in the x-z section or a Fresenl lens (in the case of theexemplary embodiment according to FIG. 3), by which the light of smallaperture a₀ is focused onto lens y.

What is claimed is:
 1. A device for optical coupling of a solid-statelaser to an optical fiber, the device comprising an anamorphic lenssystem disposed between an output face of the solid-state laser and aninput face of the optical fiber for converting differing apertures ofthe solid-state laser into essentially identical apertures at the inputface, the lens system including an anamorphic first lens mounted on theoutput face and a second lens mounted on the input face.
 2. The deviceas recited in claim 1 wherein the second lens is spherical.
 3. Thedevice as recited in claim 1 wherein the first lens is an asphericallens.
 4. The device as recited in claim 3 wherein the first lens is aFresnel lens.
 5. The device as recited in claim 1 wherein the first lensincludes two crossed cylindrical partial lenses, at least one of thepartial lenses being a Fresnel lens.
 6. The device as recited in claim 1further comprising a spacer layer disposed between the first lens andthe output face, the spacer layer corresponding to a predeterminedmagnification in a direction of a larger aperture of the differingapertures.
 7. A method of making a device for optical coupling of asolid-state laser to an optical fiber, the method comprising: providingan anamorphic lens system disposed between an output face of thesolid-state laser and an input face of the optical fiber for convertingdiffering apertures of the solid-state laser into essentially identicalapertures at the input face, the lens system including an anamorphicfirst lens mounted on the output face and a second lens mounted on theinput face; and producing the first and second lenses using a dry-resisttechnique.
 8. The method as recited in claim 7 further comprising:providing at least one spacer layer disposed between the first lens andthe output face, the spacer layer corresponding to a predeterminedmagnification in a direction of a larger aperture of the differingapertures; providing the anamorphic lens system with at least oneantireflection layer; and producing the at least one spacer layer andthe at least one antireflection layer using the dry-resist technique. 9.A method of making a device for optical coupling of a solid-state laserto an optical fiber, the method comprising: providing an anamorphic lenssystem disposed between an output face of the solid-state laser and aninput face of the optical fiber for converting differing apertures inmain sections of the solid-state laser into essentially identicalapertures at the input face, the lens system including an anamorphicfirst lens mounted on the output face and a second lens mounted on theinput face; and producing the first and second lenses using additivelithography.
 10. The method as recited in claim 9 wherein the additivelithography includes electron-beam lithography.
 11. The method asrecited in claim 7 further comprising producing the first and secondlenses using additive lithography.
 12. The method as recited in claim 11wherein the additive lithography includes electron-beam lithography. 13.The method as recited in claim 7 further comprising: providing at leastone spacer layer disposed between the first lens and the output face,the spacer layer corresponding to a predetermined magnification in adirection of a larger aperture of the differing apertures; providing theanamorphic lens system with at least one antireflection layer; andproducing the at least one spacer layer and the at least oneantireflection layer using additive lithography.
 14. The method asrecited in claim 9 further comprising: providing at least one spacerlayer disposed between the first lens and the output face, the spacerlayer corresponding to a predetermined magnification in a direction of alarger aperture of the differing apertures; providing the anamorphiclens system with at least one antireflection layer; and producing the atleast one spacer layer and the at least one antireflection layer usingadditive lithography.